Cellular samples are for some applications the preferred choice of screening in drug discovery research, potentially overtaking more traditional approaches that include animal models. The cellular samples may be used to detect specific cellular pathways of chemical compounds, therapeutic proteins, synthetic ribonucleic acid (siRNA) agents and other structures of interest. Insights from these samples could enable more efficient discovery of effective drugs compared to non-cell-based samples, thus saving time and costs as well as the need for future secondary screens. However, cellular samples usually vary from one another in terms of their usability for screening. For example, in order to perform screening on cellular samples they must exhibit certain viability. Several parameters provide an indication of a selected sample's viability such as size, form, color, and the presence of certain types of molecules. To avoid the waste of valuable resources needed for the screening of cell samples like man hours, equipment, chemical substances and compounds, the separation or sorting of usable cellular samples from non-usable ones is thus desired prior to performing any screening process. The sorting of cellular samples is done by cell sorters known in the art, which are usually adapted to receive a mixture of, e.g., cellular samples and which partition the mixture into separate cell samples according to cell types for further individual processing. Such a separated cellular sample may be a single cell or a group of cells of the same type. In particular, cellular samples may refer to any of the following: Xenopus oocytes, zebrafish larvae or embryos, pollen, cells, cell-clusters, or to any other cellular matter.
Cell sorters, which are also referred to as flow cytometers, are guiding the mixture into a nozzle which generates a jet of liquid having suspended therein the cellular samples such that individual cellular samples of the mixture are ejected from the nozzle outlet. The nozzle outlet is positioned in relation to a laser source in a manner such that ejected cellular samples may pass laser light emitted by the laser source. At least some of the ejected cellular samples interact with the ejected cellular samples causing scattering of the laser light and fluorescening of at least some of the ejected cellular samples. The photons of the fluorescening light and the scattered laser light are collected by photomultipliers. The multiplied photons pertaining to fluorescening light and scattered laser light are subsequently analyzed cytometrically to determine according to predetermined criteria if there are cellular samples for which additional examination is desirable and which are sorted accordingly. To enable the sorting of the individual cell samples, the jet of fluid at the nozzle outlet is formed into droplets containing the individual cell samples, wherein the droplets are electrically charged. The droplets, and thus the individual cell samples, become sortable towards separate collection vials by applying a static electric field according to preselected criteria after the individual cell samples pass the laser light. Alternatively, to sort individual cellular samples, collection vials may be moved into or out of the jet of fluid, or puffs of air can be used to respectively guide individual cellular samples into designated vials according to the preselected criteria. The latter method which employs puffs of air is implemented by the COPAS Sorting Platform. The throughput of such cell sorters can be as high as 100,000 cells per second and numerous measurable parameters are available with the above explained systems.
Patent document US 2002/033939 discloses such a cell sorter for analyzing and dispensing objects larger than about 70 μm in diameter. The cell sorter implements a flow cytometer having a fluidic switch arrangement for diverting a portion of a sample stream in response to detector signals. The cell sorter is particularly adapted for dispensing multicellular test organisms like nematodes or large microspheres for use in screening large libraries of potential pharmaceutical agents. Hydrodynamic focusing is used to center and align the objects in the flow cell. The objects pass through a sensing zone where optical or other characteristics of the objects are detected. The detector signals are processed and used to operate a fluidic switch that is located downstream from the sensing zone. The fluid stream containing the detected objects emerges from the flow cell into air where a fluid stream controlled by the fluidic switch diverts portions of the stream containing no sample objects or sample objects not meeting predetermined at least one criterion. The non-diverted sample stream deposits selected sample objects into a plurality of containers. To ensure reliable analysis of the samples, the throughput of the above outlined systems may have to be reduced down to approximately 10'000 individual cells per seconds. However, fluorescence markers may cause damages to the samples and under certain circumstances the number of parameters that can be determined when employing fluorescence-based procedures is limited. For example, the parameters that may be determined for opaque cells may be limited to the cell's diameter and optical density, since only forward and side scattered laser light may be measurable.
The above-outlined cell sorter does not have any storage capabilities and does not offer a controllable sample-removal mechanism to a subsequent apparatus for performing further treatment and/or analysis of the sorted cells. In addition, these cell sorters may not be adapted to handle and sort samples having diameters in the millimeter range such that the samples, which may for example refer to Xenopus laevis oocyte and Zebrafish embryos, have to be sorted manually, which is an arduous and laborious task.